INTEGRATED CIRCUITS P87C51RA2/RB2/RC2/RD2

Transcription

1 INTEGRATED CIRCUITS 80C51 8-bit microcontroller family 8KB/16KB/32KB/64KB OTP, 512B/512B/512B/1KB RAM, low voltage (2.7 to 5.5 V), low power, high speed (30/33 MHz) Supersedes data of 2002 Oct Jan 24

2 DESCRIPTION The devices are Single-Chip 8-Bit Microcontrollers manufactured in an advanced CMOS process and are derivatives of the 80C51 microcontroller family. The instruction set is 100% compatible with the 80C51 instruction set. The devices support 6-clock/12-clock mode selection by programming an OTP bit (OX2) using parallel programming. In addition, an SFR bit (X2) in the clock control register (CKCON) also selects between 6-clock/12-clock mode. The devices also have four 8-bit I/O ports, three 16-bit timer/event counters, a multi-source, four-priority-level, nested interrupt structure, an enhanced UART and on-chip oscillator and timing circuits. The added features of the make it a powerful microcontroller for applications that require pulse width modulation, high-speed I/O and up/down counting capabilities such as motor control. FEATURES 80C51 Central Processing Unit 8 kbytes OTP (87C51RA2) 16 kbytes OTP (87C51RB2) 32 kbytes OTP (87C51RC2) 64 kbytes OTP (87C51RD2) 512 byte RAM (87C51RA2/RB2/RC2) 1 kbyte RAM (87C51RD2) Boolean processor Fully static operation Low voltage (2.7 V to 5.5 V at 16 MHz) operation 12-clock operation with selectable 6-clock operation (via software or via parallel programmer) Memory addressing capability Up to 64 kbytes ROM and 64 kbytes RAM Power control modes: Clock can be stopped and resumed Idle mode Power-down mode CMOS and TTL compatible Two speed ranges at V CC = 5 V 0 to 30 MHz with 6-clock operation 0 to 33 MHz with 12-clock operation Parallel programming with 87C51 compatible hardware interface to programmer RAM expandable externally to 64 kbytes Programmable Counter Array (PCA) PWM Capture/compare PLCC, LQFP, or DIP package Extended temperature ranges Dual Data Pointers Security bits (3 bits) Encryption array - 64 bytes Seven interrupt sources 4 interrupt priority levels Four 8-bit I/O ports Full-duplex enhanced UART Framing error detection Automatic address recognition Three 16-bit timers/counters T0, T1 (standard 80C51) and additional T2 (capture and compare) Programmable clock-out pin Asynchronous port reset Low EMI (inhibit ALE, slew rate controlled outputs, and 6-clock mode) Wake-up from Power Down by an external interrupt 2003 Jan

3 SELECTION TABLE Type Memory Timers Serial Interfaces RAM ROM OTP Flash # of Timers PWM PCA WD UART I 2C CAN SPI ADC bits/ch. I/O Pins Interrupts (Ext.)/Levels Program Security Default Clock Rate Optional Clock Rate Reset active low/high? Max. Freq. at 6-clk / 12-clk (MHz) Freq. Range at 3V (MHz) Freq. Range at 5V (MHz) P87C51RD2 1K 64K (2)/4 12-clk 6-clk H 30/ /33 P87C51RC2 512B 32K (2)/4 12-clk 6-clk H 30/ /33 P87C51RB2 512B 16K (2)/4 12-clk 6-clk H 30/ /33 P87C51RA2 512B 8K (2)/4 12-clk 6-clk H 30/ /33 ORDERING INFORMATION PHILIPS (EXCEPT NORTH AMERICA) PART ORDER NUMBER PART MARKING OTP MEMORY RAM TEMPERATURE RANGE ( C) VOLTAGE RANGE DWG # AND PACKAGE P87C51RA2BA 8 KB 512B 0 to +70, PLCC 2.7 to 5.5 V SOT187-2 P87C51RA2FA 8 KB 512B 40 to +85, PLCC 2.7 to 5.5 V SOT187-2 P87C51RA2BBD 8 KB 512B 0 to +70, LQFP 2.7 to 5.5 V SOT389-1 P87C51RB2BA 16 KB 512B 0 to +70, PLCC 2.7 to 5.5 V SOT187-2 P87C51RB2FA 16 KB 512B 40 to +85, PLCC 2.7 to 5.5 V SOT187-2 P87C51RB2BBD 16 KB 512B 0 to +70, LQFP 2.7 to 5.5 V SOT389-1 P87C51RB2BN 16 KB 512B 0 to +70, DIP to 5.5 V SOT129-1 P87C51RB2FN 16 KB 512B 40 to +85, DIP to 5.5 V SOT129-1 P87C51RC2BA 32 KB 512B 0 to +70, PLCC 2.7 to 5.5 V SOT187-2 P87C51RC2FA 32 KB 512B 40 to +85, PLCC 2.7 to 5.5 V SOT187-2 P87C51RC2BBD 32 KB 512B 0 to +70, LQFP 2.7 to 5.5 V SOT389-1 P87C51RC2BN 32 KB 512B 0 to +70, DIP to 5.5 V SOT129-1 P87C51RC2FN 32 KB 512B 40 to +85, DIP to 5.5 V SOT129-1 P87C51RD2BA 64 KB 1 KB 0 to +70, PLCC 2.7 to 5.5 V SOT187-2 P87C51RD2FA 64 KB 1 KB 40 to +85, PLCC 2.7 to 5.5 V SOT187-2 P87C51RD2BBD 64 KB 1 KB 0 to +70, LQFP 2.7 to 5.5 V SOT389-1 P87C51RD2FBD 64 KB 1 KB 40 to +85, LQFP 2.7 to 5.5 V SOT389-1 P87C51RD2BN 64 KB 1 KB 0 to +70, DIP to 5.5 V SOT Jan 24 3

4 BLOCK DIAGRAM 1 ACCELERATED 80C51 CPU (12-CLK MODE, 6-CLK MODE) 8K / 16K / 32K / 64 KBYTE CODE OTP 512 / 1024 BYTE DATA RAM PORT 3 CONFIGURABLE I/Os FULL-DUPLEX ENHANCED UART TIMER 0 TIMER 1 TIMER 2 PORT 2 CONFIGURABLE I/Os PORT 1 CONFIGURABLE I/Os PROGRAMMABLE COUNTER ARRAY (PCA) WATCHDOG TIMER PORT 0 CONFIGURABLE I/Os CRYSTAL OR RESONATOR OSCILLATOR su Jan 24 4

5 BLOCK DIAGRAM (CPU-ORIENTED) P0.0 P0.7 P2.0 P2.7 PORT 0 DRIVERS PORT 2 DRIVERS V CC V SS RAM ADDR REGISTER RAM PORT 0 LATCH PORT 2 LATCH OTP MEMORY 8 B REGISTER ACC STACK POINTER TMP2 TMP1 PROGRAM ADDRESS REGISTER ALU BUFFER PSW SFRs TIMERS P.C.A. PC INCRE- MENTER 8 16 PROGRAM COUNTER PSEN ALE EAV PP RST TIMING AND CONTROL INSTRUCTION REGISTER DPTR S MULTIPLE PD PORT 1 LATCH PORT 3 LATCH OSCILLATOR XTAL1 XTAL2 PORT 1 DRIVERS PORT 3 DRIVERS P1.0 P1.7 P3.0 P3.7 SU Jan 24 5

6 LOGIC SYMBOL Plastic Leaded Chip Carrier V CC V SS XTAL PORT 0 ADDRESS AND DATA BUS LCC SECONDARY FUNCTIONS RxD TxD INT0 INT1 T0 T1 WR RD XTAL2 RST EA/V PP PSEN ALE/PROG PORT 3 PORT 2 PORT 1 T2 T2EX ADDRESS BUS SU01672 Pin Function 1 NIC* 2 P1.0/T2 3 P1.1/T2EX 4 P1.2/ECI 5 P1.3/CEX0 6 P1.4/CEX1 7 P1.5/CEX2 8 P1.6/CEX3 9 P1.7/CEX4 10 RST 11 P3.0/RxD 12 NIC* 13 P3.1/TxD 14 P3.2/INT0 15 P3.3/INT Pin Function 16 P3.4/T0 17 P3.5/T1 18 P3.6/WR 19 P3.7/RD 20 XTAL2 21 XTAL1 22 V SS 23 NIC* 24 P2.0/A8 25 P2.1/A9 26 P2.2/A10 27 P2.3/A11 28 P2.4/A12 29 P2.5/A13 30 P2.6/A14 29 Pin Function 31 P2.7/A15 32 PSEN 33 ALE/PROG 34 NIC* 35 EA/V PP 36 P0.7/AD7 37 P0.6/AD6 38 P0.5/AD5 39 P0.4/AD4 40 P0.3/AD3 41 P0.2/AD2 42 P0.1/AD1 43 P0.0/AD0 44 V CC PINNING * NO INTERNAL CONNECTION SU00023 Plastic Dual In-Line Package T2/P1.0 T2EX/P V CC P0.0/AD0 Plastic Quad Flat Pack ECI/P1.2 CEX0/P P0.1/AD1 P0.2/AD CEX1/P P0.3/AD3 LQFP CEX2/P P0.4/AD4 CEX3/P P0.5/AD CEX4/P P0.6/AD6 RST 9 32 P0.7/AD RxD/P TxD/P INT0/P INT1/P T0/P T1/P WR/P RD/P XTAL2 18 XTAL1 19 V SS DUAL IN-LINE PACKAGE EA/V PP ALE/PROG PSEN P2.7/A15 P2.6/A14 P2.5/A13 P2.4/A12 P2.3/A11 P2.2/A10 P2.1/A9 P2.0/A8 Pin Function 1 P1.5/CEX2 2 P1.6/CEX3 3 P1.7/CEX4 4 RST 5 P3.0/RxD 6 NIC* 7 P3.1/TxD 8 P3.2/INT0 9 P3.3/INT1 10 P3.4/T0 11 P3.5/T1 12 P3.6/WR 13 P3.7/RD 14 XTAL2 15 XTAL1 * NO INTERNAL CONNECTION Pin Function 16 V SS 17 NIC* 18 P2.0/A8 19 P2.1/A9 20 P2.2/A10 21 P2.3/A11 22 P2.4/A12 23 P2.5/A13 24 P2.6/A14 25 P2.7/A15 26 PSEN 27 ALE/PROG 28 NIC* 29 EA/V PP 30 P0.7/AD7 Pin Function 31 P0.6/AD6 32 P0.5/AD5 33 P0.4/AD4 34 P0.3/AD3 35 P0.2/AD2 36 P0.1/AD1 37 P0.0/AD0 38 V CC 39 NIC* 40 P1.0/T2 41 P1.1/T2EX 42 P1.2/ECI 43 P1.3/CEX0 44 P1.4/CEX1 SU01400 SU Jan 24 6

7 PIN DESCRIPTIONS MNEMONIC PIN NUMBER PDIP PLCC LQFP TYPE NAME AND FUNCTION V SS I Ground: 0 V reference. V CC I Power Supply: This is the power supply voltage for normal, idle, and power-down operation. P I/O Port 0: Port 0 is an open-drain, bidirectional I/O port. Port 0 pins that have 1s written to them float and can be used as high-impedance inputs. Port 0 is also the multiplexed low-order address and data bus during accesses to external program and data memory. In this application, it uses strong internal pull-ups when emitting 1s. P1.0 P , 1 3 I/O Port 1: Port 1 is an 8-bit bidirectional I/O port with internal pull-ups on all pins. Port 1 pins that have 1s written to them are pulled high by the internal pull-ups and can be used as inputs. As inputs, port 1 pins that are externally pulled low will source current because of the internal pull-ups. (See DC Electrical Characteristics: I IL ). Alternate functions for Port 1 include: I/O T2 (P1.0): Timer/Counter 2 external count input/clockout (see Programmable Clock-Out) I T2EX (P1.1): Timer/Counter 2 Reload/Capture/Direction Control I ECI (P1.2): External Clock Input to the PCA I/O CEX0 (P1.3): Capture/Compare External I/O for PCA module I/O CEX1 (P1.4): Capture/Compare External I/O for PCA module I/O CEX2 (P1.5): Capture/Compare External I/O for PCA module I/O CEX3 (P1.6): Capture/Compare External I/O for PCA module I/O CEX4 (P1.7): Capture/Compare External I/O for PCA module 4 P2.0 P I/O Port 2: Port 2 is an 8-bit bidirectional I/O port with internal pull-ups. Port 2 pins that have 1s written to them are pulled high by the internal pull-ups and can be used as inputs. As inputs, port 2 pins that are externally being pulled low will source current because of the internal pull-ups. (See DC Electrical Characteristics: I IL ). Port 2 emits the high-order address byte during fetches from external program memory and during accesses to external data memory that use 16-bit addresses In this application, it uses strong internal pull-ups when emitting 1s. During accesses to external data memory that use 8-bit addresses port 2 emits the contents of the P2 special function register. P3.0 P , , 7 13 I/O Port 3: Port 3 is an 8-bit bidirectional I/O port with internal pull-ups. Port 3 pins that have 1s written to them are pulled high by the internal pull-ups and can be used as inputs. As inputs, port 3 pins that are externally being pulled low will source current because of the pull-ups. (See DC Electrical Characteristics: I IL ). Port 3 also serves the special features of the, as listed below: I RxD (P3.0): Serial input port O TxD (P3.1): Serial output port I INT0 (P3.2): External interrupt I INT1 (P3.3): External interrupt I T0 (P3.4): Timer 0 external input I T1 (P3.5): Timer 1 external input O WR (P3.6): External data memory write strobe O RD (P3.7): External data memory read strobe RST I Reset: A high on this pin for two machine cycles while the oscillator is running, resets the device. An internal resistor to V SS permits a power-on reset using only an external capacitor to V CC. ALE O Address Latch Enable: Output pulse for latching the low byte of the address during an access to external memory. In normal operation, ALE is emitted twice every machine cycle, and can be used for external timing or clocking. Note that one ALE pulse is skipped during each access to external data memory. ALE can be disabled by setting SFR auxiliary.0. With this bit set, ALE will be active only during a MOVX instruction Jan 24 7

8 MNEMONIC PDIP PIN NUMBER PLCC LQFP TYPE NAME AND FUNCTION PSEN O Program Store Enable: The read strobe to external program memory. When executing code from the external program memory, PSEN is activated twice each machine cycle, except that two PSEN activations are skipped during each access to external data memory. PSEN is not activated during fetches from internal program memory. EA/V PP I External Access Enable/Programming Supply Voltage: EA must be externally held low to enable the device to fetch code from external program memory locations. If EA is held high, the device executes from internal program memory. The value on the EA pin is latched when RST is released and any subsequent changes have no effect. This pin also receives the programming supply voltage (V PP ) during programming. XTAL I Crystal 1: Input to the inverting oscillator amplifier and input to the internal clock generator circuits. XTAL O Crystal 2: Output from the inverting oscillator amplifier. NOTE: To avoid latch-up effect at power-on, the voltage on any pin (other than V PP ) must not be higher than V CC V or less than V SS 0.5 V Jan 24 8

9 SPECIAL FUNCTION REGISTERS SYMBOL DESCRIPTION DIRECT BIT ADDRESS, SYMBOL, OR ALTERNATIVE PORT FUNCTION RESET ADDRESS MSB LSB VALUE ACC* Accumulator E0H E7 E6 E5 E4 E3 E2 E1 E0 00H AUXR# Auxiliary 8EH EXTRAM AO xxxxxx00b AUXR1# Auxiliary 1 A2H GF2 0 DPS xxxxxxx0b B* B register F0H F7 F6 F5 F4 F3 F2 F1 F0 00H CCAP0H# Module 0 Capture High FAH xxxxxxxxb CCAP1H# Module 1 Capture High FBH xxxxxxxxb CCAP2H# Module 2 Capture High FCH xxxxxxxxb CCAP3H# Module 3 Capture High FDH xxxxxxxxb CCAP4H# Module 4 Capture High FEH xxxxxxxxb CCAP0L# Module 0 Capture Low EAH xxxxxxxxb CCAP1L# Module 1 Capture Low EBH xxxxxxxxb CCAP2L# Module 2 Capture Low ECH xxxxxxxxb CCAP3L# Module 3 Capture Low EDH xxxxxxxxb CCAP4L# Module 4 Capture Low EEH xxxxxxxxb CCAPM0# Module 0 Mode DAH ECOM CAPP CAPN MAT TOG PWM ECCF x b CCAPM1# Module 1 Mode DBH ECOM CAPP CAPN MAT TOG PWM ECCF x b CCAPM2# Module 2 Mode DCH ECOM CAPP CAPN MAT TOG PWM ECCF x b CCAPM3# Module 3 Mode DDH ECOM CAPP CAPN MAT TOG PWM ECCF x b CCAPM4# Module 4 Mode DEH ECOM CAPP CAPN MAT TOG PWM ECCF x b DF DE DD DC DB DA D9 D8 CCON*# PCA Counter Control D8H CF CR CCF4 CCF3 CCF2 CCF1 CCF0 00x00000B CH# PCA Counter High F9H 00H CKCON# Clock control 8FH X2 x b CL# PCA Counter Low E9H 00H CMOD# PCA Counter Mode D9H CIDL WDTE CPS1 CPS0 ECF 00xxx000B DPTR: Data Pointer (2 bytes) DPH Data Pointer High 83H 00H DPL Data Pointer Low 82H 00H AF AE AD AC AB AA A9 A8 IE* Interrupt Enable 0 A8H EA EC ET2 ES ET1 EX1 ET0 EX0 00H BF BE BD BC BB BA B9 B8 IP* Interrupt Priority B8H PPC PT2 PS PT1 PX1 PT0 PX0 x b B7 B6 B5 B4 B3 B2 B1 B0 IPH# Interrupt Priority High B7H PPCH PT2H PSH PT1H PX1H PT0H PX0H x b P0* Port 0 80H AD7 AD6 AD5 AD4 AD3 AD2 AD1 AD0 FFH P1* Port 1 90H CEX4 CEX3 CEX2 CEX1 CEX0 ECI T2EX T2 FFH A7 A6 A5 A4 A3 A2 A1 A0 P2* Port 2 A0H AD15 AD14 AD13 AD12 AD11 AD10 AD9 AD8 FFH B7 B6 B5 B4 B3 B2 B1 B0 P3* Port 3 B0H RD WR T1 T0 INT1 INT0 TxD RxD FFH PCON# 1 Power Control 87H SMOD1 SMOD0 POF GF1 GF0 PD IDL 00xxx000B * SFRs are bit addressable. # SFRs are modified from or added to the 80C51 SFRs. Reserved bits. 1. Reset value depends on reset source Jan 24 9

10 SPECIAL FUNCTION REGISTERS (Continued) SYMBOL DESCRIPTION DIRECT ADDRESS BIT ADDRESS, SYMBOL, OR ALTERNATIVE PORT FUNCTION MSB LSB RESET VALUE D7 D6 D5 D4 D3 D2 D1 D0 PSW* Program Status Word D0H CY AC F0 RS1 RS0 OV F1 P B RCAP2H# Timer 2 Capture High CBH 00H RCAP2L# Timer 2 Capture Low CAH 00H SADDR# Slave Address A9H 00H SADEN# Slave Address Mask B9H 00H SBUF Serial Data Buffer 99H xxxxxxxxb 9F 9E 9D 9C 9B 9A SCON* Serial Control 98H SM0/FE SM1 SM2 REN TB8 RB8 TI RI 00H SP Stack Pointer 81H 07H 8F 8E 8D 8C 8B 8A TCON* Timer Control 88H TF1 TR1 TF0 TR0 IE1 IT1 IE0 IT0 00H CF CE CD CC CB CA C9 C8 T2CON* Timer 2 Control C8H TF2 EXF2 RCLK TCLK EXEN2 TR2 C/T2 CP/RL2 00H T2MOD# Timer 2 Mode Control C9H T2OE DCEN xxxxxx00b TH0 Timer High 0 8CH 00H TH1 Timer High 1 8DH 00H TH2# Timer High 2 CDH 00H TL0 Timer Low 0 8AH 00H TL1 Timer Low 1 8BH 00H TL2# Timer Low 2 CCH 00H TMOD Timer Mode 89H GATE C/T M1 M0 GATE C/T M1 M0 00H WDTRST Watchdog Timer Reset A6H * SFRs are bit addressable. # SFRs are modified from or added to the 80C51 SFRs. Reserved bits. OSCILLATOR CHARACTERISTICS XTAL1 and XTAL2 are the input and output, respectively, of an inverting amplifier. The pins can be configured for use as an on-chip oscillator. To drive the device from an external clock source, XTAL1 should be driven while XTAL2 is left unconnected. Minimum and maximum high and low times specified in the data sheet must be observed. This device is configured at the factory to operate using 12 clock periods per machine cycle, referred to in this datasheet as 12-clock mode. It may be optionally configured on commercially available parallel programming equipment or via software to operate at 6 clocks per machine cycle, referred to in this datasheet as 6-clock mode. (This yields performance equivalent to twice that of standard 80C51 family devices). Also see next page Jan 24 10

11 CLOCK CONTROL REGISTER (CKCON) This device allows control of the 6-clock/12-clock mode by means of both an SFR bit (X2) and an OTP bit. The OTP clock control bit OX2, when programmed (6-clock mode), supersedes the X2 bit (CKCON.0). The CKCON register is shown below in Figure 1. CKCON Address = 8Fh Reset Value = x b Not Bit Addressable X2 BIT SYMBOL FUNCTION CKCON.7 Reserved. CKCON.6 Reserved. CKCON.5 Reserved. CKCON.4 Reserved. CKCON.3 Reserved. CKCON.2 Reserved. CKCON.1 Reserved. CKCON.0 X2 CPU clock; 1 = 6 clocks for each machine cycle, 0 = 12 clocks for each machine cycle SU01689 Figure 1. Clock control (CKCON) register Also please note that the clock divider applies to the serial port for modes 0 & 2 (fixed baud rate modes). This is because modes 1 & 3 (variable baud rate modes) use either Timer 1 or Timer 2. Below is the truth table for the CPU clock mode. Table 1. OX2 clock mode bit (can only be set by parallel programmer) X2 bit (CKCON.0) CPU clock mode erased 0 12-clock mode (default) erased 1 6-clock mode programmed X 6-clock mode RESET A reset is accomplished by holding the RST pin high for at least two machine cycles (12 oscillator periods in 6-clock mode, or 24 oscillator periods in 12-clock mode), while the oscillator is running. To ensure a good power-on reset, the RST pin must be high long enough to allow the oscillator time to start up (normally a few milliseconds) plus two machine cycles. At power-on, the voltage on V CC and RST must come up at the same time for a proper start-up. Ports 1, 2, and 3 will asynchronously be driven to their reset condition when a voltage above V IH1 (min.) is applied to RST. The value on the EA pin is latched when RST is deasserted and has no further effect Jan 24 11

12 LOW POWER MODES Stop Clock Mode The static design enables the clock speed to be reduced down to 0 MHz (stopped). When the oscillator is stopped, the RAM and Special Function Registers retain their values. This mode allows step-by-step utilization and permits reduced system power consumption by lowering the clock frequency down to any value. For lowest power consumption the Power Down mode is suggested. Idle Mode In the idle mode (see Table 2), the CPU puts itself to sleep while all of the on-chip peripherals stay active. The instruction to invoke the idle mode is the last instruction executed in the normal operating mode before the idle mode is activated. The CPU contents, the on-chip RAM, and all of the special function registers remain intact during this mode. The idle mode can be terminated either by any enabled interrupt (at which time the process is picked up at the interrupt service routine and continued), or by a hardware reset which starts the processor in the same manner as a power-on reset. Power-Down Mode To save even more power, a Power Down mode (see Table 2) can be invoked by software. In this mode, the oscillator is stopped and the instruction that invoked Power Down is the last instruction executed. The on-chip RAM and Special Function Registers retain their values down to 2 V and care must be taken to return V CC to the minimum specified operating voltages before the Power Down Mode is terminated. Either a hardware reset or external interrupt can be used to exit from Power Down. Reset redefines all the SFRs but does not change the on-chip RAM. An external interrupt allows both the SFRs and the on-chip RAM to retain their values. To properly terminate Power Down, the reset or external interrupt should not be executed before V CC is restored to its normal operating level and must be held active long enough for the oscillator to restart and stabilize (normally less than 10 ms). With an external interrupt, INT0 and INT1 must be enabled and configured as level-sensitive. Holding the pin low restarts the oscillator but bringing the pin back high completes the exit. Once the interrupt is serviced, the next instruction to be executed after RETI will be the one following the instruction that put the device into Power Down. POWER-ON FLAG The Power-On Flag (POF) is set by on-chip circuitry when the V CC level on the rises from 0 to 5 V. The POF bit can be set or cleared by software allowing a user to determine if the reset is the result of a power-on or a warm start after powerdown. The V CC level must remain above 3 V for the POF to remain unaffected by the V CC level. Design Consideration When the idle mode is terminated by a hardware reset, the device normally resumes program execution, from where it left off, up to two machine cycles before the internal reset algorithm takes control. On-chip hardware inhibits access to internal RAM in this event, but access to the port pins is not inhibited. To eliminate the possibility of an unexpected write when Idle is terminated by reset, the instruction following the one that invokes Idle should not be one that writes to a port pin or to external memory. ONCE Mode The ONCE ( On-Circuit Emulation ) Mode facilitates testing and debugging of systems without the device having to be removed from the circuit. The ONCE Mode is invoked by: 1. Pull ALE low while the device is in reset and PSEN is high; 2. Hold ALE low as RST is deactivated. While the device is in ONCE Mode, the Port 0 pins go into a float state, and the other port pins and ALE and PSEN are weakly pulled high. The oscillator circuit remains active. While the device is in this mode, an emulator or test CPU can be used to drive the circuit. Normal operation is restored when a normal reset is applied. Programmable Clock-Out A 50% duty cycle clock can be programmed to come out on P1.0. This pin, besides being a regular I/O pin, has two alternate functions. It can be programmed: 1. to input the external clock for Timer/Counter 2, or 2. to output a 50% duty cycle clock ranging from 61 Hz to 4 MHz at a 16 MHz operating frequency in 12-clock mode (122 Hz to 8 MHz in 6-clock mode). To configure the Timer/Counter 2 as a clock generator, bit C/T2 (in T2CON) must be cleared and bit T20E in T2MOD must be set. Bit TR2 (T2CON.2) also must be set to start the timer. The Clock-Out frequency depends on the oscillator frequency and the reload value of Timer 2 capture registers (RCAP2H, RCAP2L) as shown in this equation: Oscillator Frequency n (65536 RCAP2H, RCAP2L) n = 2 in 6-clock mode 4 in 12-clock mode Where (RCAP2H,RCAP2L) = the content of RCAP2H and RCAP2L taken as a 16-bit unsigned integer. In the Clock-Out mode Timer 2 roll-overs will not generate an interrupt. This is similar to when it is used as a baud-rate generator. It is possible to use Timer 2 as a baud-rate generator and a clock generator simultaneously. Note, however, that the baud-rate and the Clock-Out frequency will be the same. Table 2. External Pin Status During Idle and Power-Down Mode MODE PROGRAM MEMORY ALE PSEN PORT 0 PORT 1 PORT 2 PORT 3 Idle Internal 1 1 Data Data Data Data Idle External 1 1 Float Data Address Data Power-down Internal 0 0 Data Data Data Data Power-down External 0 0 Float Data Data Data 2003 Jan 24 12

13 TIMER 0 AND TIMER 1 OPERATION Timer 0 and Timer 1 The Timer or Counter function is selected by control bits C/T in the Special Function Register TMOD. These two Timer/Counters have four operating modes, which are selected by bit-pairs (M1, M0) in TMOD. Modes 0, 1, and 2 are the same for both Timers/Counters. Mode 3 is different. The four operating modes are described in the following text. Mode 0 Putting either Timer into Mode 0 makes it look like an 8048 Timer, which is an 8-bit Counter with a divide-by-32 prescaler. Figure 3 shows the Mode 0 operation. In this mode, the Timer register is configured as a 13-bit register. As the count rolls over from all 1s to all 0s, it sets the Timer interrupt flag TFn. The counted input is enabled to the Timer when TRn = 1 and either GATE = 0 or INTn = 1. (Setting GATE = 1 allows the Timer to be controlled by external input INTn, to facilitate pulse width measurements). TRn is a control bit in the Special Function Register TCON (Figure 4). The 13-bit register consists of all 8 bits of THn and the lower 5 bits of TLn. The upper 3 bits of TLn are indeterminate and should be ignored. Setting the run flag (TRn) does not clear the registers. Mode 0 operation is the same for Timer 0 as for Timer 1. There are two different GATE bits, one for Timer 1 (TMOD.7) and one for Timer 0 (TMOD.3). Mode 1 Mode 1 is the same as Mode 0, except that the Timer register is being run with all 16 bits. Mode 2 Mode 2 configures the Timer register as an 8-bit Counter (TLn) with automatic reload, as shown in Figure 5. Overflow from TLn not only sets TFn, but also reloads TLn with the contents of THn, which is preset by software. The reload leaves THn unchanged. Mode 2 operation is the same for Timer 0 as for Timer 1. Mode 3 Timer 1 in Mode 3 simply holds its count. The effect is the same as setting TR1 = 0. Timer 0 in Mode 3 establishes TL0 and TH0 as two separate counters. The logic for Mode 3 on Timer 0 is shown in Figure 6. TL0 uses the Timer 0 control bits: C/T, GATE, TR0, and TF0 as well as pin INT0. TH0 is locked into a timer function (counting machine cycles) and takes over the use of TR1 and TF1 from Timer 1. Thus, TH0 now controls the Timer 1 interrupt. Mode 3 is provided for applications requiring an extra 8-bit timer on the counter. With Timer 0 in Mode 3, an 80C51 can look like it has three Timer/Counters. When Timer 0 is in Mode 3, Timer 1 can be turned on and off by switching it out of and into its own Mode 3, or can still be used by the serial port as a baud rate generator, or in fact, in any application not requiring an interrupt. TMOD Address = 89H Reset Value = 00H Not Bit Addressable GATE C/T M1 M0 GATE C/T M1 M0 BIT SYMBOL FUNCTION TMOD.3/ GATE Gating control when set. Timer/Counter n is enabled only while INTn pin is high and TMOD.7 TRn control pin is set. when cleared Timer n is enabled whenever TRn control bit is set. TMOD.2/ C/T Timer or Counter Selector cleared for Timer operation (input from internal system clock.) TMOD.6 Set for Counter operation (input from Tn input pin). M1 M0 OPERATING Timer: TLn serves as 5-bit prescaler bit Timer/Counter: THn and TLn are cascaded; there is no prescaler bit auto-reload Timer/Counter: THn holds a value which is to be reloaded into TLn each time it overflows. 1 1 (Timer 0) TL0 is an 8-bit Timer/Counter controlled by the standard Timer 0 control bits. TH0 is an 8-bit timer only controlled by Timer 1 control bits. 1 1 (Timer 1) Timer/Counter 1 stopped. TIMER 1 TIMER 0 SU01580 Figure 2. Timer/Counter 0/1 Mode Control (TMOD) Register 2003 Jan 24 13

14 OSC d* C/T = 0 C/T = 1 TLn (5 Bits) THn (8 Bits) TFn Interrupt Tn Pin Control TRn Timer n Gate bit INTn Pin *d = 6 in 6-clock mode; d = 12 in 12-clock mode. SU01618 Figure 3. Timer/Counter 0/1 Mode 0: 13-Bit Timer/Counter TCON Address = 88H Reset Value = 00H Bit Addressable TF1 TR1 TF0 TR0 IE1 IT1 IE0 IT0 BIT SYMBOL FUNCTION TCON.7 TF1 Timer 1 overflow flag. Set by hardware on Timer/Counter overflow. Cleared by hardware when processor vectors to interrupt routine, or clearing the bit in software. TCON.6 TR1 Timer 1 Run control bit. Set/cleared by software to turn Timer/Counter on/off. TCON.5 TF0 Timer 0 overflow flag. Set by hardware on Timer/Counter overflow. Cleared by hardware when processor vectors to interrupt routine, or by clearing the bit in software. TCON.4 TR0 Timer 0 Run control bit. Set/cleared by software to turn Timer/Counter on/off. TCON.3 IE1 Interrupt 1 Edge flag. Set by hardware when external interrupt edge detected. Cleared when interrupt processed. TCON.2 IT1 Interrupt 1 type control bit. Set/cleared by software to specify falling edge/low level triggered external interrupts. TCON.1 IE0 Interrupt 0 Edge flag. Set by hardware when external interrupt edge detected. Cleared when interrupt processed. TCON.0 IT0 Interrupt 0 Type control bit. Set/cleared by software to specify falling edge/low level triggered external interrupts. SU01516 Figure 4. Timer/Counter 0/1 Control (TCON) Register 2003 Jan 24 14

15 OSC d* Tn Pin C/T = 0 C/T = 1 Control TLn (8 Bits) TFn Interrupt TRn Reload Timer n Gate bit INTn Pin THn (8 Bits) *d = 6 in 6-clock mode; d = 12 in 12-clock mode. SU01619 Figure 5. Timer/Counter 0/1 Mode 2: 8-Bit Auto-Reload OSC d* T0 Pin C/T = 0 C/T = 1 Control TL0 (8 Bits) TF0 Interrupt TR0 Timer 0 Gate bit INT0 Pin OSC d* TH0 (8 Bits) TF1 Interrupt Control *d = 6 in 6-clock mode; d = 12 in 12-clock mode. TR1 SU01620 Figure 6. Timer/Counter 0 Mode 3: Two 8-Bit Counters 2003 Jan 24 15

16 TIMER 2 OPERATION Timer 2 Timer 2 is a 16-bit Timer/Counter which can operate as either an event timer or an event counter, as selected by C/T2 in the special function register T2CON (see Figure 1). Timer 2 has three operating modes: Capture, Auto-reload (up or down counting), and Baud Rate Generator, which are selected by bits in the T2CON as shown in Table 3. Capture Mode In the capture mode there are two options which are selected by bit EXEN2 in T2CON. If EXEN2=0, then timer 2 is a 16-bit timer or counter (as selected by C/T2 in T2CON) which, upon overflowing sets bit TF2, the timer 2 overflow bit. This bit can be used to generate an interrupt (by enabling the Timer 2 interrupt bit in the IE register). If EXEN2= 1, Timer 2 operates as described above, but with the added feature that a 1- to -0 transition at external input T2EX causes the current value in the Timer 2 registers, TL2 and TH2, to be captured into registers RCAP2L and RCAP2H, respectively. In addition, the transition at T2EX causes bit EXF2 in T2CON to be set, and EXF2 like TF2 can generate an interrupt (which vectors to the same location as Timer 2 overflow interrupt. The Timer 2 interrupt service routine can interrogate TF2 and EXF2 to determine which event caused the interrupt). The capture mode is illustrated in Figure 2 (There is no reload value for TL2 and TH2 in this mode. Even when a capture event occurs from T2EX, the counter keeps on counting T2EX pin transitions or osc/6 pulses (osc/12 in 12-clock mode).). Auto-Reload Mode (Up or Down Counter) In the 16-bit auto-reload mode, Timer 2 can be configured (as either a timer or counter [C/T2 in T2CON]) then programmed to count up or down. The counting direction is determined by bit DCEN (Down Counter Enable) which is located in the T2MOD register (see Figure 3). When reset is applied the DCEN=0 which means Timer 2 will default to counting up. If DCEN bit is set, Timer 2 can count up or down depending on the value of the T2EX pin. Figure 4 shows Timer 2 which will count up automatically since DCEN=0. In this mode there are two options selected by bit EXEN2 in T2CON register. If EXEN2=0, then Timer 2 counts up to 0FFFFH and sets the TF2 (Overflow Flag) bit upon overflow. This causes the Timer 2 registers to be reloaded with the 16-bit value in RCAP2L and RCAP2H. The values in RCAP2L and RCAP2H are preset by software means. If EXEN2=1, then a 16-bit reload can be triggered either by an overflow or by a 1-to-0 transition at input T2EX. This transition also sets the EXF2 bit. The Timer 2 interrupt, if enabled, can be generated when either TF2 or EXF2 are 1. In Figure 5 DCEN=1 which enables Timer 2 to count up or down. This mode allows pin T2EX to control the direction of count. When a logic 1 is applied at pin T2EX Timer 2 will count up. Timer 2 will overflow at 0FFFFH and set the TF2 flag, which can then generate an interrupt, if the interrupt is enabled. This timer overflow also causes the 16-bit value in RCAP2L and RCAP2H to be reloaded into the timer registers TL2 and TH2. When a logic 0 is applied at pin T2EX this causes Timer 2 to count down. The timer will underflow when TL2 and TH2 become equal to the value stored in RCAP2L and RCAP2H. Timer 2 underflow sets the TF2 flag and causes 0FFFFH to be reloaded into the timer registers TL2 and TH2. The external flag EXF2 toggles when Timer 2 underflows or overflows. This EXF2 bit can be used as a 17th bit of resolution if needed. The EXF2 flag does not generate an interrupt in this mode of operation. (MSB) (LSB) TF2 EXF2 RCLK TCLK EXEN2 TR2 C/T2 CP/RL2 Symbol Position Name and Significance TF2 T2CON.7 Timer 2 overflow flag set by a Timer 2 overflow and must be cleared by software. TF2 will not be set when either RCLK or TCLK = 1. EXF2 T2CON.6 Timer 2 external flag set when either a capture or reload is caused by a negative transition on T2EX and EXEN2 = 1. When Timer 2 interrupt is enabled, EXF2 = 1 will cause the CPU to vector to the Timer 2 interrupt routine. EXF2 must be cleared by software. EXF2 does not cause an interrupt in up/down counter mode (DCEN = 1). RCLK T2CON.5 Receive clock flag. When set, causes the serial port to use Timer 2 overflow pulses for its receive clock in modes 1 and 3. RCLK = 0 causes Timer 1 overflow to be used for the receive clock. TCLK T2CON.4 Transmit clock flag. When set, causes the serial port to use Timer 2 overflow pulses for its transmit clock in modes 1 and 3. TCLK = 0 causes Timer 1 overflows to be used for the transmit clock. EXEN2 T2CON.3 Timer 2 external enable flag. When set, allows a capture or reload to occur as a result of a negative transition on T2EX if Timer 2 is not being used to clock the serial port. EXEN2 = 0 causes Timer 2 to ignore events at T2EX. TR2 T2CON.2 Start/stop control for Timer 2. A logic 1 starts the timer. C/T2 T2CON.1 Timer or counter select. (Timer 2) 0 = Internal timer (OSC/6 in 6-clock mode or OSC/12 in 12-clock mode) 1 = External event counter (falling edge triggered). CP/RL2 T2CON.0 Capture/Reload flag. When set, captures will occur on negative transitions at T2EX if EXEN2 = 1. When cleared, auto-reloads will occur either with Timer 2 overflows or negative transitions at T2EX when EXEN2 = 1. When either RCLK = 1 or TCLK = 1, this bit is ignored and the timer is forced to auto-reload on Timer 2 overflow. SU01251 Figure 1. Timer/Counter 2 (T2CON) Control Register 2003 Jan 24 16

17 Table 3. Timer 2 Operating Modes RCLK + TCLK CP/RL2 TR2 MODE bit Auto-reload bit Capture 1 X 1 Baud rate generator X X 0 (off) OSC n* C/T2 = 0 TL2 (8 BITS) TH2 (8 BITS) TF2 T2 Pin C/T2 = 1 Control Transition Detector TR2 Capture Timer 2 Interrupt RCAP2L RCAP2H T2EX Pin EXF2 Control EXEN2 SU01252 * n = 6 in 6-clock mode, or 12 in 12-clock mode. Figure 2. Timer 2 in Capture Mode T2MOD Address = 0C9H Reset Value = XXXX XX00B Not Bit Addressable T2OE DCEN Symbol Bit Function Not implemented, reserved for future use.* T2OE Timer 2 Output Enable bit. DCEN Down Count Enable bit. When set, this allows Timer 2 to be configured as an up/down counter. * User software should not write 1s to reserved bits. These bits may be used in future 8051 family products to invoke new features. In that case, the reset or inactive value of the new bit will be 0, and its active value will be 1. The value read from a reserved bit is indeterminate. SU00729 Figure 3. Timer 2 Mode (T2MOD) Control Register 2003 Jan 24 17

18 OSC n* C/T2 = 0 TL2 (8 BITS) TH2 (8 BITS) T2 PIN C/T2 = 1 CONTROL TR2 RELOAD TRANSITION DETECTOR RCAP2L RCAP2H TF2 TIMER 2 INTERRUPT T2EX PIN EXF2 CONTROL EXEN2 SU01253 * n = 6 in 6-clock mode, or 12 in 12-clock mode. Figure 4. Timer 2 in Auto-Reload Mode (DCEN = 0) (DOWN COUNTING RELOAD VALUE) FFH FFH TOGGLE EXF2 OSC n* C/T2 = 0 TL2 TH2 OVERFLOW TF2 INTERRUPT T2 PIN C/T2 = 1 CONTROL TR2 COUNT DIRECTION 1 = UP 0 = DOWN RCAP2L RCAP2H * n = 6 in 6-clock mode, or 12 in 12-clock mode. (UP COUNTING RELOAD VALUE) Figure 5. Timer 2 Auto Reload Mode (DCEN = 1) T2EX PIN SU Jan 24 18

19 Timer 1 Overflow n = 1 in 6-clock mode n = 2 in 12-clock mode 2 OSC n C/T2 = 0 TL2 (8-bits) TH2 (8-bits) SMOD RCLK T2 Pin C/T2 = 1 TR2 Control Reload RX Clock TCLK Transition Detector RCAP2L RCAP2H 16 TX Clock T2EX Pin EXF2 Timer 2 Interrupt EXEN2 Control Note availability of additional external interrupt. SU01629 Figure 6. Timer 2 in Baud Rate Generator Mode Table 4. Timer 2 Generated Commonly Used Baud Rates Baud Rate Timer 2 12-clock 6-clock Osc Freq RCAP2H RCAP2L mode mode 375 k 750 k 12 MHz FF FF 9.6 k 19.2 k 12 MHz FF D9 4.8 k 9.6 k 12 MHz FF B2 2.4 k 4.8 k 12 MHz FF k 2.4 k 12 MHz FE C MHz FB 1E MHz F2 AF MHz FD 8F MHz F9 57 Baud Rate Generator Mode Bits TCLK and/or RCLK in T2CON (Table 4) allow the serial port transmit and receive baud rates to be derived from either Timer 1 or Timer 2. When TCLK= 0, Timer 1 is used as the serial port transmit baud rate generator. When TCLK= 1, Timer 2 is used as the serial port transmit baud rate generator. RCLK has the same effect for the serial port receive baud rate. With these two bits, the serial port can have different receive and transmit baud rates one generated by Timer 1, the other by Timer 2. Figure 6 shows the Timer 2 in baud rate generation mode. The baud rate generation mode is like the auto-reload mode,in that a rollover in TH2 causes the Timer 2 registers to be reloaded with the 16-bit value in registers RCAP2H and RCAP2L, which are preset by software. The baud rates in modes 1 and 3 are determined by Timer 2 s overflow rate given below: Modes 1 and 3 Baud Rates Timer 2 Overflow Rate 16 The timer can be configured for either timer or counter operation. In many applications, it is configured for timer operation (C/T2=0). Timer operation is different for Timer 2 when it is being used as a baud rate generator. Usually, as a timer it would increment every machine cycle (i.e., 1 / 6 the oscillator frequency in 6-clock mode, 1 / 12 the oscillator frequency in 12-clock mode). As a baud rate generator, it increments at the oscillator frequency in 6-clock mode ( OSC / 2 in 12-clock mode). Thus the baud rate formula is as follows: Modes 1 and 3 Baud Rates = Oscillator Frequency [n* [65536 (RCAP2H, RCAP2L)]] * n = 16 in 6-clock mode 32 in 12-clock mode Where: (RCAP2H, RCAP2L)= The content of RCAP2H and RCAP2L taken as a 16-bit unsigned integer. The Timer 2 as a baud rate generator mode shown in Figure 6, is valid only if RCLK and/or TCLK = 1 in T2CON register. Note that a rollover in TH2 does not set TF2, and will not generate an interrupt. Thus, the Timer 2 interrupt does not have to be disabled when Timer 2 is in the baud rate generator mode. Also if the EXEN2 (T2 external enable flag) is set, a 1-to-0 transition in T2EX (Timer/counter 2 trigger input) will set EXF2 (T2 external flag) but will not cause a reload from (RCAP2H, RCAP2L) to (TH2,TL2). Therefore when Timer 2 is in use as a baud rate generator, T2EX can be used as an additional external interrupt, if needed Jan 24 19

20 When Timer 2 is in the baud rate generator mode, one should not try to read or write TH2 and TL2. As a baud rate generator, Timer 2 is incremented every state time (osc/2) or asynchronously from pin T2; under these conditions, a read or write of TH2 or TL2 may not be accurate. The RCAP2 registers may be read, but should not be written to, because a write might overlap a reload and cause write and/or reload errors. The timer should be turned off (clear TR2) before accessing the Timer 2 or RCAP2 registers. Table 4 shows commonly used baud rates and how they can be obtained from Timer 2. Summary of Baud Rate Equations Timer 2 is in baud rate generating mode. If Timer 2 is being clocked through pin T2 (P1.0) the baud rate is: Baud Rate Timer 2 Overflow Rate 16 If Timer 2 is being clocked internally, the baud rate is: f Baud Rate OSC [n* [65536 (RCAP2H, RCAP2L)]] * n = 16 in 6-clock mode 32 in 12-clock mode Where f OSC = Oscillator Frequency To obtain the reload value for RCAP2H and RCAP2L, the above equation can be rewritten as: f RCAP2H, RCAP2L OSC n* Baud Rate Timer/Counter 2 Set-up Except for the baud rate generator mode, the values given for T2CON do not include the setting of the TR2 bit. Therefore, bit TR2 must be set, separately, to turn the timer on. see Table 5 for set-up of Timer 2 as a timer. Also see Table 6 for set-up of Timer 2 as a counter. Table 5. Timer 2 as a Timer MODE INTERNAL CONTROL (Note 1) T2CON EXTERNAL CONTROL (Note 2) 16-bit Auto-Reload 00H 08H 16-bit Capture 01H 09H Baud rate generator receive and transmit same baud rate 34H 36H Receive only 24H 26H Transmit only 14H 16H Table 6. Timer 2 as a Counter MODE INTERNAL CONTROL (Note 1) TMOD EXTERNAL CONTROL (Note 2) 16-bit 02H 0AH Auto-Reload 03H 0BH NOTES: 1. Capture/reload occurs only on timer/counter overflow. 2. Capture/reload occurs on timer/counter overflow and a 1-to-0 transition on T2EX (P1.1) pin except when Timer 2 is used in the baud rate generator mode Jan 24 20

21 FULL-DUPLEX ENHANCED UART Standard UART operation The serial port is full duplex, meaning it can transmit and receive simultaneously. It is also receive-buffered, meaning it can commence reception of a second byte before a previously received byte has been read from the register. (However, if the first byte still hasn t been read by the time reception of the second byte is complete, one of the bytes will be lost.) The serial port receive and transmit registers are both accessed at Special Function Register SBUF. Writing to SBUF loads the transmit register, and reading SBUF accesses a physically separate receive register. The serial port can operate in 4 modes: Mode 0: Serial data enters and exits through RxD. TxD outputs the shift clock. 8 bits are transmitted/received (LSB first). The baud rate is fixed at 1/12 the oscillator frequency in 12-clock mode or 1/6 the oscillator frequency in 6-clock mode. Mode 1: 10 bits are transmitted (through TxD) or received (through RxD): a start bit (0), 8 data bits (LSB first), and a stop bit (1). On receive, the stop bit goes into RB8 in Special Function Register SCON. The baud rate is variable. Mode 2: 11 bits are transmitted (through TxD) or received (through RxD): start bit (0), 8 data bits (LSB first), a programmable 9th data bit, and a stop bit (1). On Transmit, the 9th data bit (TB8 in SCON) can be assigned the value of 0 or 1. Or, for example, the parity bit (P, in the PSW) could be moved into TB8. On receive, the 9th data bit goes into RB8 in Special Function Register SCON, while the stop bit is ignored. The baud rate is programmable to either 1/32 or 1/64 the oscillator frequency in 12-clock mode or 1/16 or 1/32 the oscillator frequency in 6-clock mode. Mode 3: 11 bits are transmitted (through TxD) or received (through RxD): a start bit (0), 8 data bits (LSB first), a programmable 9th data bit, and a stop bit (1). In fact, Mode 3 is the same as Mode 2 in all respects except baud rate. The baud rate in Mode 3 is variable. In all four modes, transmission is initiated by any instruction that uses SBUF as a destination register. Reception is initiated in Mode 0 by the condition RI = 0 and REN = 1. Reception is initiated in the other modes by the incoming start bit if REN = 1. Multiprocessor Communications Modes 2 and 3 have a special provision for multiprocessor communications. In these modes, 9 data bits are received. The 9th one goes into RB8. Then comes a stop bit. The port can be programmed such that when the stop bit is received, the serial port interrupt will be activated only if RB8 = 1. This feature is enabled by setting bit SM2 in SCON. A way to use this feature in multiprocessor systems is as follows: When the master processor wants to transmit a block of data to one of several slaves, it first sends out an address byte which identifies the target slave. An address byte differs from a data byte in that the 9th bit is 1 in an address byte and 0 in a data byte. With SM2 = 1, no slave will be interrupted by a data byte. An address byte, however, will interrupt all slaves, so that each slave can examine the received byte and see if it is being addressed. The addressed slave will clear its SM2 bit and prepare to receive the data bytes that will be coming. The slaves that weren t being addressed leave their SM2s set and go on about their business, ignoring the coming data bytes. SM2 has no effect in Mode 0, and in Mode 1 can be used to check the validity of the stop bit. In a Mode 1 reception, if SM2 = 1, the receive interrupt will not be activated unless a valid stop bit is received. Serial Port Control Register The serial port control and status register is the Special Function Register SCON, shown in Figure 7. This register contains not only the mode selection bits, but also the 9th data bit for transmit and receive (TB8 and RB8), and the serial port interrupt bits (TI and RI). Baud Rates The baud rate in Mode 0 is fixed: Mode 0 Baud Rate = Oscillator Frequency / 12 (12-clock mode) or / 6 (6-clock mode). The baud rate in Mode 2 depends on the value of bit SMOD in Special Function Register PCON. If SMOD = 0 (which is the value on reset), and the port pins in 12-clock mode, the baud rate is 1/64 the oscillator frequency. If SMOD = 1, the baud rate is 1/32 the oscillator frequency. In 6-clock mode, the baud rate is 1/32 or 1/16 the oscillator frequency, respectively. Mode 2 Baud Rate = 2 SMOD n (Oscillator Frequency) Where: n = 64 in 12-clock mode, 32 in 6-clock mode The baud rates in Modes 1 and 3 are determined by the Timer 1 or Timer 2 overflow rate. Using Timer 1 to Generate Baud Rates When Timer 1 is used as the baud rate generator (T2CON.RCLK = 0, T2CON.TCLK = 0), the baud rates in Modes 1 and 3 are determined by the Timer 1 overflow rate and the value of SMOD as follows: Mode 1, 3 Baud Rate = 2 SMOD n (Timer 1 Overflow Rate) Where: n = 32 in 12-clock mode, 16 in 6-clock mode The Timer 1 interrupt should be disabled in this application. The Timer itself can be configured for either timer or counter operation, and in any of its 3 running modes. In the most typical applications, it is configured for timer operation, in the auto-reload mode (high nibble of TMOD = 0010B). In that case the baud rate is given by the formula: Mode 1, 3 Baud Rate = 2 SMOD Oscillator Frequency n 12 [256 (TH1)] Where: n = 32 in 12-clock mode, 16 in 6-clock mode One can achieve very low baud rates with Timer 1 by leaving the Timer 1 interrupt enabled, and configuring the Timer to run as a 16-bit timer (high nibble of TMOD = 0001B), and using the Timer 1 interrupt to do a 16-bit software reload. Figure 8 lists various commonly used baud rates and how they can be obtained from Timer Jan 24 21

22 SCON Address = 98H Reset Value = 00H Bit Addressable SM0 SM1 SM2 REN TB8 RB8 TI RI Where SM0, SM1 specify the serial port mode, as follows: SM0 SM1 Mode Description Baud Rate shift register f OSC /12 (12-clock mode) or f OSC /6 (6-clock mode) bit UART variable bit UART f OSC /64 or f OSC /32 (12-clock mode) or f OSC /32 or f OSC /16 (6-clock mode) bit UART variable SM2 Enables the multiprocessor communication feature in Modes 2 and 3. In Mode 2 or 3, if SM2 is set to 1, then Rl will not be activated if the received 9th data bit (RB8) is 0. In Mode 1, if SM2=1 then RI will not be activated if a valid stop bit was not received. In Mode 0, SM2 should be 0. REN Enables serial reception. Set by software to enable reception. Clear by software to disable reception. TB8 The 9th data bit that will be transmitted in Modes 2 and 3. Set or clear by software as desired. RB8 In Modes 2 and 3, is the 9th data bit that was received. In Mode 1, it SM2=0, RB8 is the stop bit that was received. In Mode 0, RB8 is not used. TI Transmit interrupt flag. Set by hardware at the end of the 8th bit time in Mode 0, or at the beginning of the stop bit in the other modes, in any serial transmission. Must be cleared by software. RI Receive interrupt flag. Set by hardware at the end of the 8th bit time in Mode 0, or halfway through the stop bit time in the other modes, in any serial reception (except see SM2). Must be cleared by software. SU01626 Figure 7. Serial Port Control (SCON) Register Baud Rate Mode 12-clock mode 6-clock mode f OSC SMOD Timer 1 C/T Mode Reload Value Mode 0 Max 1.67 MHz 3.34 MHz 20 MHz X X X X Mode 2 Max 625 k 1250 k 20 MHz 1 X X X Mode 1, 3 Max k k 20 MHz FFH Mode 1, k 38.4 k MHz FDH 9.6 k 19.2 k MHz FDH 4.8 k 9.6 k MHz FAH 2.4 k 4.8 k MHz F4H 1.2 k 2.4 k MHz E8H MHz DH MHz H MHz FEEBH Figure 8. Timer 1 Generated Commonly Used Baud Rates More About Mode 0 Serial data enters and exits through RxD. TxD outputs the shift clock. 8 bits are transmitted/received: 8 data bits (LSB first). The baud rate is fixed a 1/12 the oscillator frequency (12-clock mode) or 1/6 the oscillator frequency (6-clock mode). Figure 9 shows a simplified functional diagram of the serial port in Mode 0, and associated timing. Transmission is initiated by any instruction that uses SBUF as a destination register. The write to SBUF signal at S6P2 also loads a 1 into the 9th position of the transmit shift register and tells the TX Control block to commence a transmission. The internal timing is such that one full machine cycle will elapse between write to SBUF and activation of SEND. SEND enables the output of the shift register to the alternate output function line of P3.0 and also enable SHIFT CLOCK to the alternate output function line of P3.1. SHIFT CLOCK is low during S3, S4, and S5 of every machine cycle, and high during S6, S1, and S2. At S6P2 of every machine cycle in which SEND is active, the contents of the transmit shift are shifted to the right one position. As data bits shift out to the right, zeros come in from the left. When the MSB of the data byte is at the output position of the shift register, then the 1 that was initially loaded into the 9th position, is just to the left of the MSB, and all positions to the left of that contain zeros. This condition flags the TX Control block to do one last shift and then deactivate SEND and set T1. Both of these actions occur at S1P1 of the 10th machine cycle after write to SBUF. Reception is initiated by the condition REN = 1 and R1 = 0. At S6P2 of the next machine cycle, the RX Control unit writes the bits to the receive shift register, and in the next clock phase activates RECEIVE. RECEIVE enable SHIFT CLOCK to the alternate output function line of P3.1. SHIFT CLOCK makes transitions at S3P1 and S6P1 of every machine cycle. At S6P2 of every machine cycle in which RECEIVE is active, the contents of the receive shift register are 2003 Jan 24 22